Microelectronic devices, such as memory devices and microprocessors, typically include a microelectronic die encased in a protective covering. The die can include memory cells, processor circuits, interconnecting circuitry and/or other functional features. The die also typically includes an array of very small bond-pads electrically coupled to the functional features. When the die is packaged, the bond-pads are coupled to leads, solder ball-pads or other types of terminals for operatively coupling the microelectronic dies to buses, circuits and/or other microelectronic devices.
Several different techniques have been developed for packaging microelectronic dies. The dies, for example, can be incorporated into individual packages, mounted with other components in hybrid or multiple chip modules, or connected directly to a printed circuit board or other types of substrates. When a die is incorporated into an individual package, the bond-pads on the die are typically coupled to a lead frame, and the die is covered or otherwise sealed from the environment. When the die is attached directly to a printed circuit board or another type of substrate, the bond-pads on the die are typically coupled to corresponding contact elements on the substrate using wire-bond lines, ball grid arrays and other techniques. The dies that are mounted directly to the substrates are generally Chip Scale Package devices (CSP) or Flip Chip Bare Die devices (Flip-Chip).
CSP and Flip-Chip devices generally have one or more protective casings that encapsulate the dies and any exposed contact elements, bond-pads or wire-bond lines. The protective casings should shield the die and the other components on the substrate from environmental factors (e.g., moisture), electrical interference, and mechanical shocks. The protective casings are accordingly robust elements that protect the sensitive components of a microelectronic device. The protective casings are generally composed of plastics, ceramics, or thermosetting materials.
One conventional technique for fabricating the protective casings involves placing the die in a cavity of a mold, and then injecting a thermosetting material into the cavity. The thermosetting material flows over the die on one side of the substrate until it fills the cavity, and then the thermosetting material is cured so that it hardens into a suitable protective casing for protecting the die. The protective casing should not have any voids over the die because contaminants from the molding process or environmental factors outside of the mold could damage the die. The thermosetting material, moreover, should not cover a ball-pad array on the substrate or damage any electrical connections between the die and the substrate. Therefore, the thermosetting material should be molded in a manner that avoids (a) producing voids in the protective casing, (b) covering certain portions of the substrate, and (c) displacing or otherwise damaging any wire-bond lines or solder joints between the die and the substrate.
One drawback of forming protective casings is that the thermosetting material may leak between the substrate and a mold assembly as the thermosetting material fills the mold. The thermosetting material generally leaks because the substrates can have small surface asperities and/or be warped. Such leaking of the thermosetting material can cover ball-pad arrays and/or adhere to the mold. When the thermosetting material covers a portion of the ball-pad array, the packaged device is typically rejected because it cannot be electrically coupled to a module. Additionally, the molds must be cleaned periodically to remove the thermosetting material that adheres to the mold. Cleaning the molds, however, is difficult because they operate at approximately 180° C., and thus they are difficult to handle and they must also be reheated after they have been cleaned. The down time for cleaning a mold can be approximately 15% of the available operating time for a molding machine. Therefore, it would be desirable to prevent the thermosetting material from leaking between the substrate and the mold.
One technique that addresses the leakage between the substrate and the mold is to cover the inside of the mold with a thin plastic film. This technique, however, is time consuming because the mold must generally be cooled to a temperature at which it can be handled, and it is difficult to attach the plastic film to the mold without creating wrinkles in the plastic film. Moreover, if the plastic film has wrinkles, the resulting protective casings may have surface asperities that are either unsightly or impair the performance of the protective casing. Therefore, covering the inside of a mold with a thin plastic film is not a good solution for preventing the thermosetting material from leaking between the substrate and the mold.